1. Field of the Invention
The present invention relates to a method of patterning the surface of a substrate with at least one crystalline compound.
2. Description of the Related Art
In the field of microelectronics there is a constant need to develop smaller device elements that can be reproduced conveniently and inexpensively with a lowest possible failure rate. Modern digital integrated circuits are based on field-effect transistors (FET), which rely on an electric field to control the conductivity of a “channel” in a semiconductor material. Organic field-effect transistors (OFET) allow the production of flexible or unbreakable substrates for integrated circuits having large active areas. As OFETs enable the production of complex circuits they have a wide area of potential application (e.g. in driver circuits of pixel displays).
Lithographic techniques for the manufacture of integrated circuits (IC) are well known in the art. However, the smaller the device, the more difficult it is to manufacture and, thus, the more expensive it becomes. Moreover, in the production of semiconductors on a molecular scale, lithographic approaches may fail owing to lithographic constraints imposed by resolution and alignment. It is therefore desirable to produce ICs by techniques which use a driving force that causes atoms to assemble in the desired fashion (self-assembling of electronic circuitry).
Different methods for the self-assembly of micro-objects onto substrates are known. A first technique is the fluidic self-assembly, wherein the device units or “blocks” are shaped to match receptor sites or “holes” that have been etched into the substrate. The blocks are suspended in a carrier liquid that is dispensed over the substrate. The blocks fall towards the receptor sites and, with the assistance of fluid flow and/or acoustic vibration, self-orient into the holes or are removed from the substrate to be recirculated. The driving force behind this assembly process is gravity. Development of the fluidic self-assembly is based on capillary force. Benjamin R. Martin, Donna C. Furnange, Thomas N. Jackson, Thomas E. Mallouk, and Theresa S. Mayer describe in Adv. Funct. Mater. 2001, 11, 381-386, the self-alignment of patterned wafers using capillary forces at a water-air interface. Those capillary interactions were used to align glass wafers with silicon-wafers. A silica surface was patterned with gold millimeter-scale borders enclosing micrometer-scale gold alignment marks in the center. The gold portions of the pattern were rendered hydrophobic with a self-assembled monolayer formed from long-chain alkanthiols. Water dropped on the surface selectively wetted the hydrophilic silica regions defined by the pattern. These confined droplets drove the alignment when two wafers with complementary hydrophobic/hydrophilic patterns were pressed together.
A further self-assembly technique makes use of patterned surfaces. Fengqiu Fan and Kathleen J. Stebe describe in Langmuir, 2004, 20, 3062-3067, the assembly of colloidal particles by evaporation on surfaces with patterned hydrophobicity. To obtain chemical modifications the surface of an Au-coated silicon wafer substrate was patterned into hydrophobic/hydrophilic regions using microcontact printing. To prepare hydrophilic self-assembled monolayers (SAM) a solution of HS(CH2)15COOH was brought into contact with the gold surface using a poly(dimethylsiloxane) (PDMS) stamp. Subsequently, the substrates were immersed in a solution of HS(CH2)17CH3 to cover the remainder of the surface with a hydrophobic SAM. Colloidal polystyrene particles, optionally functionalized with positively charged amidine groups or negatively charged sulfate groups, were dispersed into water; drops containing the suspended particles were then placed on the surface of the substrate to study their adsorption behaviour.
U.S. Pat. No. 6,887,332 B1 teaches a method for forming a thin film on a surface of a substrate having thereon a patterned underlayer of a self-assembled monolayer. The underlayer is prepared by coating the surface with an organic molecular species having head functional groups capable of interacting with the surface of the substrate and tail functional groups for chemical differentiation of patterned and unpatterned regions of the coated surface by microstamping. The thin film is deposited on the underlayer by a solution-based charges. Micro-objects that are also functionalized with charge through surface modification can be patterned into selected regions through electrostatic interactions.
A further self-assembly technique makes use of patterned topography. Description: The key strategy of this process is the dewetting of a colloid dispersion on a substrate that has been patterned with an array of templates (such as cylindrical holes). When this dispersion is allowed to dewet slowly, the capillary force leads to an assembly of the colloidal particles in the templates.
A further self-assembly technique makes use of the patterning of objects through applied electric or magnetic fields. The electrical or magnetic contacts of the substrates were prefabricated. By adding an external electric or magnetic field, objects could be aligned or placed in certain region on the substrates.
According to the microfluidic self-assembly technique, solutions containing 1-D objects can fill the microchannels via capillary action or liquid flow. Upon evaporation of the solvent or during liquid flow, the objects can be aligned onto substrates.
U.S. Pat. No. 6,828,582 B1 describes a thin film transistor comprising a gate electrode, a gate insulation film, a source electrode, a drain electrode, a semiconductor film and a protection film, stacked on a substrate, wherein the semiconductor film is composed of an aggregate of organic semiconductor molecules, and the organic semiconductor molecules of the semiconductor film formed in a gate electrode-projected region on a surface of the insulation film have a higher orientation order than that of the semiconductor film formed outside the region. The organic semiconductor film is formed by selectively disposing a self-assembled monolayer film on the surface of the insulation film and then forming the semiconductor film thereon by making use of the orientation order of the self assembled monolayer of the insulation film. A water-repellent monolayer film having a carbon chain partly terminated with a fluorine or hydrogen atom is used as the self-assembled monolayer film.
WO 2004/114371 A2 discloses a compound used to form a self-assembled monolayer, especially for a semiconductor component, being characterised by a molecular group capable of π-π interaction with other similar compounds and/or other different compounds for the stabilisation of the monolayer.
WO 2005/001952 A1 discloses a compound for forming a self-organizing monolayer, particularly for forming a layer structure for an organic field effect transistor, characterized by: a) at least one anchor group for binding the molecule to a substrate, particularly an electrode material; b) at least one dielectric group, and; c) at least one semiconducting group. Y. Zhao and J. Fang describe in Langmuir, 2006 (published in the internet) a method for positioning and aligning self-assembled tubules of 1,2-bis(tricosa-10,12-diynoyl)-syn-glycero-3-phosphocholine by withdrawing a patterned Au substrate from a lipid tubule solution.
U.S. 2004/0061104 A1 discloses a method for making an integrated circuit (IC) in which organic semiconductor crystallites function as active channels of organic semiconductor devices. The method includes providing a substrate with a surface that has a preselected pattern of adhesion sites located thereon and capable of adhering crystallites of an organic semiconductor. This document does not teach the use of microcontact printing (“stamping”) to form a semiconductor pattern on the surface of the substrate.
The patterning methods according to the prior art show at least one of the following disadvantages:                high cost (e.g. fluidic self-assembly techniques needs lithography or etching to make substrate patterns)        low throughput (especially substrate topography-templated assembly is a slow process)        complexity (e.g. patterning by applied electric or magnetic fields)        no general applicability (e.g. high demands on surface properties of micro-objects; e.g.: fluidic assembly, electrostatic self-assembly, etc. Generally, it is problematic to functionalize organic single crystals without changing their properties).        little control on inter-object spacing and/or on object orientation (especially by self-assembly through electrostatic forces)        